Anyway – for the Wayfarer the spinnaker numbering rule is perfectly clear.
This is an extract from the WIC Rules 2009
30.7 Sail numbers on spinnaker. Shall be shown on the front, or on both sides, at approximately half height. The numbers shall be of contrasting colour to the sail and not less than 300 (11¾”) in height nor 200 (7.7/8″) in width (except for numeral 1) nor 45 (1¾”) in thickness.